The goals of the pediatric HIV research effort are to conduct translational research aimed at developing improved therapies for children with HIV infection and HIV-associated malignancies, to understand the pathogenesis of pediatric HIV disease, and to study the impact of pediatric HIV disease on the patients and their families. Both antiretroviral and immunologic approaches for HIV therapy are currently being investigated. During FY 2005, this work was performed by Drs. Rohan Hazra, Steven Zeichner, Lauren Wood, Alexandra Freeman, Carol Worrell, Lori Wiener, Pam Wolters, and other members of the Pediatric Working Group in HAMB. A protocol investigating the use of tenofovir, a nucleotide analog reverse transcriptase inhibitor, in HIV-infected children who have failed previous regimens and have multidrug resistant virus began to yield published results. The primary aim of the study was to obtain the pharmacokinetic and safety data needed to use the drug as part of highly active antiretroviral therapy (HAART) in the pediatric population. Secondary objectives included assessment of the long-term clinical, virologic, and immunologic responses to tenofovir-containing HAART and studies on the evolution of drug resistance in this population. We have published the tenofovir pharmacokinetic data obtained in the study, and a follow-up study has been accepted for publication. The major toxicity associated with tenofovir in children in our study was significant decreases in bone mineral density. A follow-up study exploring the use of tenofovir in children, especially its effect on bone metabolism, is actively accruing patients. Another protocol that is actively accruing patients involves a therapeutic drug monitoring approach to antiretroviral therapy in children. In this study, we obtain viral resistance genotypes and inferred viral resistance phenotypes, design an optimal HAART regimen based on that data and the patient's history, and establish a target plasma level for the protease inhibitor component of the antiretroviral regimen. After initiation of therapy, we measure plasma drug levels, compare them to the target levels, and increase the dose of the drug if the levels do not exceed the target levels. The protocol also includes extensive studies of adherence to antiretroviral therapy. Subjects in both the studies of tenofovir and therapeutic drug monitoring are experiencing surprisingly good clinical, virologic, and immunologic responses. Together these studies are leading us to alter our approach to the treatment of patients with multidrug resistant virus. The protease inhibitors have been used in children at the NCI longer than anywhere else, which offers us an unparalleled opportunity to study the long-term effects of HAART including a protease inhibitor and, in particular, the ability of such regimens to effect immune reconstitution. Results suggest that some patients on long-term therapy have immunologic improvement without sustained drops in their viral load. We are also examining some of the toxicities that accompany long-term HAART. We have published a retrospective study describing the risks for lipodystrophy and dyslipidemia with long-term protease inhibitor therapy in children, identifying an association between the onset of puberty and lipodystrophy, and better viral load responses and dyslipidemia. We are now collecting data prospectively to better characterize lipodystrophy and dyslipidemia in this population with the aim to design a trial to study potential interventions for this series of toxicities in experienced patients. We have just initiated a protocol to study CNS disease in HIV-infected children in the HAART era.